Existing portable electronic appliances are mostly equipped with a hard disc drive for the purpose of data storage capacity thereof. A hard disc drive includes a hard disc for storing data and a magnetic head for recording data to and reproducing data from the hard disc. When recording data to or reproducing data from the hard disc, the magnetic head moves to a position where it is located opposite to the hard disc.
In hard disc drives, the hard disc is driven to rotate when data are recorded to or reproduced from the hard disc so that air is drawn into the space between the magnetic head and the hard disc to lift the magnetic head. Therefore, if the power supply is abruptly interrupted and the hard disc stops rotating, air is no longer drawn into the space between the magnetic head and the hard disc so that the magnetic head can come to contact the hard disc to consequently destroy the hard disc drive.
For the purpose of preventing such a problem from taking place, the hard disc drive is provided with an auto-retract feature that retracts the magnetic head to and save it in a position where it is not located opposite to the hard disc when the power supply is interrupted.
Meanwhile, if a portable electronic appliance falls while data are being recorded to or reproduced from the hard disc thereof, the magnetic head at a position located opposite to the hard disc collides with the hard disc to consequently destroy the hard disc drive.
For the purpose of preventing the hard disc drive from being destroyed due to a fall, such portable electronic appliances are equipped with a feature of preventing the hard disc drive from being destroyed, the feature being adapted to recognize a fall, turn off the power supply of the hard disc drive and retract the magnetic head to and save it in a position where it is not located opposite to the hard disc by utilizing the auto-retract feature.
As a method of recognizing a fall, there has been proposed a method of determining the size of the synthetic acceleration vector synthetically prepared by detecting the accelerations in at least three directions that are not found on a same plane and then detecting that the size of the synthetic acceleration vector is stabilized for a predetermined period of time at or near the value of 0 (see, inter alia, Patent Document 1).
With the proposed method, a free fall from a 25 cm high position that is believed to be the limit of not destroying a hard disc drive by a fall is assumed and the magnetic head is retracted to and saved in a position where it is not located opposite to the hard disc when the period of time during which the size of the synthetic acceleration vector is stabilized at or near the value of 0 is 220 milliseconds or more because then it is possible to determine that the portable electronic appliance is falling.
However, with such an arrangement, it takes about 100 milliseconds from the time when it detects a fall to the time when the operation of retracting and saving the magnetic head is completed. Additionally, for certain hard disc drives, a free fall from a position that is less than 25 cm high is the limit of not being destroyed. In view of these circumstances, it may be not permissible to spend 220 milliseconds for determining a fall for the purpose of retracting and saving the magnetic head with the above-described method. More specifically, the time period that can be spent for determining a fall is about 50 milliseconds at longest.
Meanwhile, portable electronic appliances are carried by people. Oscillations take place as they are carried. Then, as indicated by X in FIG. 1 of the accompanying drawings, there may frequently arise a situation where the size of the synthetic acceleration vector is stabilized at or near the value of 0 for about 50 milliseconds, although a fall is not taking place.
Thus, with an arrangement where the magnetic head is retracted from the position where it is located opposite to the hard disc upon detecting the size of the synthetic acceleration vector that is stabilized at or near the value of 0 for about 50 milliseconds in order to prevent the hard disc drive from being destroyed, there may frequently arise a situation where the magnetic head is retracted from the position where it is located opposite to the hard disc although the portable electronic appliance is not falling.
If the magnetic head is retracted from the position where it is located opposite to the hard disc because of an oscillation that takes place while the portable electronic appliance is being carried, the operation of recording data to or reproducing data from the hard disc is interrupted while the portable electronic appliance is being carried and operated. As the operation of recording data to or reproducing data from the hard disc is interrupted, the user of the portable electronic appliance falls into trouble. For example, the sound being produced from the portable electronic appliance may be interrupted while the user is listening to it.
The number of times of utilization of the auto-retract feature is generally limited for such hard disc drives. In other words, if a situation where the magnetic head is retracted from the position where it is located opposite to the hard disc because it is determined that the portable electronic appliance is falling although it is actually not falling, the service life of the auto-retract feature is curtailed to consequently make the hard disc drive vulnerable to destruction and allow the magnetic head to collide with the hard disc.
Additionally, the size of the synthetic acceleration vector of accelerations in three directions that are not found on the same plane becomes equal to 0 only in the case of free fall. In other words, when the portable electronic appliance falls along a slope or when it falls while it is spinning, the size of the synthetic acceleration vector does not become equal to 0. It is found to be not close to 0 but equal to a certain value smaller than the size of the original synthetic acceleration vector for a predetermined period of time as shown in FIG. 2 of the accompanying drawings. Thus, if it is so arranged as to detect a situation where the size of the synthetic acceleration vector is equal to 0 is detected to determine a fall of the portable electronic appliance, such an arrangement cannot detect a fall along a slope or a spinning fall and hence cannot prevent the hard disc drive from being destroyed. Patent Document 1: Jpn. Patent No. 3441668